A few years ago this method was suggested to me by AVS member Richard Adams, and it still works like a charm, as long as all your local DTV stations are on the UHF band.
Buy the following parts from RS:
(1) 15-1237 VHF UHF Band Splitter
(1) 15-1144 Terminator
(1) 15-1170 Inline Amp
(1) Any high bandwidth F-Type splitter/combiner
(3-4) Short RG-6 coax cables
1) Run RF from DTV antenna into Band Splitter
2) Run UHF output from Band Splitter into one input of the splitter/combiner
3) Terminate VHF output from Band Splitter
3) Run 5000 HD Modulator output into Inline amp
4) Run Inline Amp output into other input of splitter/combiner
5) Run output of splitter/combiner into TU-DST DTV input
This will let the TU-DST50/51 see the 5000 output on ch 3 or 4 (which you select in the 5000 HDTV menu), and the local DTV stations will also be seen on their UHF channel number, unless you have the Channel Mapping option turned on in the TU-DST set up menu, in which case they will show up on the remapped channel number.
'Better Living Through Modern, Expensive, Electronic Devices'